1. Field of the Invention
The present invention relates to the field of controlling an optical disc drive, and more particularly, to an adaptive focusing method and apparatus to control an optical disc drive using an adaptive focus search algorithm.
2. Description of the Related Art
An optical disc is a mass storage medium that can hold up to several tens gigabytes of binary data in an inexpensive plastic disc. Due to a large capacity of the optical disc, the optical disc is widely used as a major data storage medium in the computer field and multimedia devices.
An optical disc drive includes a large number of component parts including motors, actuators, and optical and electronic components. Many control algorithms are used to control motors and actuators to move a pickup head in a proper way so as to access a recording layer on a disc surface. One of problems of the control algorithms is to focus a laser beam onto the surface of the optical disc. More particularly, the control algorithms have a problem with detecting and adjusting an optimal focus point of the pickup head.
In conventional optical pickup heads, a laser diode is used to generate the laser beam to be radiated onto a data area (i.e., the recording layer) of the optical disc. A photo sensor, which generates opto-electrical signals representing focusing/tracking errors in the optical pickup heads, detects the laser beam reflected from the surface of the optical disc. The photo sensor includes a number of separate light sensitive areas. Some of the light sensitive areas is used to generate a focusing error signal (FES). The FES is used in servo control algorithms to detect and adjust the pickup head to correct a focusing position. A control signal of a focus actuator is referred to as a focus drive signal (FOD).
FIG. 1 is a time diagram illustrating a process including a general focusing algorithm used in the optical disc drive. At an initial stage, the pickup head moves from a central position 0.0 (0-state) toward a lowermost position (1-state). When the pickup head reaches the lowermost position (1-state), the pickup head goes to the uppermost position (2-state) to make a saw-tooth pulse movement. When the pickup head arrives at the uppermost position (2-state), the pickup head goes back to the lowermost position (1-state) and then repeats the process of going to the 2-state and back to the 1-state until the pickup head stably achieves focusing. This procedure is called a focus search. ‘FODTOPMARGN’ is a constant to define a maximum value of the FOD during a focus search and ‘FODBOTMARGN’ is a constant to define a minimum value of the FOD during the focus search. ‘FODMAGNSTEP’ and ‘FODTIMESTEP’ are constants to determine a velocity of the focus actuator, i.e., constants to define fixed distance intervals for a smallest change in the FOD and time intervals between closest changes in the FOD, respectively.
When the pickup head approaches the focus point, the FES changes like S-curves shown in FIG. 2. A center of the S-curves corresponds to the focus point of the pickup head and a number of S-curves is equal to a number of layers of an optical disc.
However, a high-density optical disc uses an objective lens having a small numerical aperture (NA) and has a small gap between the surface and a recording layer of the high-density optical disc. Thus, a focal length of the objective lens decreases and an allowance of a vertical distortion of the high-density optical disc increases. As a result, when the general focusing algorithm presented in FIG. 1 is used, the objective lens is highly likely to scratch or damage the surface of the high-density optical disc. A searching time is long until the stable focusing is attained in an initial search for a focus point and after losing the focus due to dirt on and damage to the surface of the high-density optical disc.
Documents describing conventional focusing algorithms include International Patent Application No. WO 02/59888, entitled “Optical Disk Drive with Digital Focus and Tracking Servo System”, U.S. patent application Ser. No. 6,192,010, entitled “Apparatus and Method for Adjusting Pickup Head of Optical Disc Drive to Optimal Focus Point”, and U.S. patent application Ser. No. 6,392,971, entitled “Focus control Method and Optical Disc Recording/Reproducing Apparatus”.